1. Field of the Invention
The present invention relates to a back light unit for improving a brightness and a difference of the color sense of a liquid crystal display having a direct-below-type back light and a liquid crystal display using the same.
2. Discussion of the Related Art
In general, a liquid crystal display (hereinafter referred to as “LCD”) has a characteristic of lightness, thinness, and low-power-consumption driving and thus its application scope has been widened. The LCD is used in an office automation machine and an audio/video machine in accordance with such a trend. The LCD adjusts transmitting quantity of light beam in accordance with image signal applied to a plurality of control switches arranged in a matrix form to thereby display desired pictures in a screen.
Since the LCD is not a spontaneous display apparatus, it needs a light source such as a back light. The back light for LCD has a two types such as a direct-below-type method and a light guide plate method. The direct-below-type back light has several phosphor lamps in a plane. Further direct-below-type back light has a diffusion plate installed between phosphor lamp disposed and liquid crystal panel to thereby constantly maintain a gap between liquid crystal panel and diffusion plate. The light guide plate back light has a phosphor lamp installed in an outside of the plate so that light is incident to whole surface of a liquid crystal panel by using a transparent light guide plate from the phosphor lamp.
Referring to FIG. 1, the LCD using the direct-below-type back light of a related art includes a liquid crystal panel 20 for displaying pictures and a back light unit 2 for uniformly radiating light to the liquid crystal panel 20.
The liquid crystal cells are arranged in active matrix form between upper and lower substrates 22 and 24 of the liquid crystal panel 20. Pixel electrodes and common electrodes are installed to apply electric field for each liquid crystal cell. In general, while the pixel electrode is formed for each liquid crystal cell on the lower substrate 24, that is, on the thin film transistor substrate, the common electrode is integrally formed in one body on whole surface of the upper substrate 22. Each of the pixel electrodes is connected to a thin film transistor used as a switch device. The pixel electrode drives the liquid crystal cell along with the common electrode in accordance with data signal supplied through thin film transistor to thereby display pictures corresponding to video signal.
Back light unit 2 includes a plurality of lamps 12 generating light, a lamp housing located in the lower part of the plurality of lamps 12 (or a lamp container 10 of the direct-below-type back light unit 2), a diffusion plate 14 located in the upper part of the plurality of lamps 12, and optical sheets 16 located on the diffusion plate 14.
Each of the lamps 12 includes a glass tube, an inert gas in the glass tube, and a cathode and an anode installed at opposing ends of the glass tube. The inert gas is filled up in the glass tube, and a phosphor is applied at the inner wall of the glass tube.
In the lamps 12, if AC voltage from the inverter (not shown) is applied to a high-pressure electrode (or a first envelope part H) and a low-pressure electrode (or a second envelope part L), electrons are emitted from the low-pressure electrode L and are collided with the inert gas in the glass tube whereby the quantity of electrons is increased exponentially. The current flows in the glass tube by the increase electrons, the inert gas is excited by the electrons and thus ultraviolet rays are emitted. The ultraviolet rays are collided with the phosphor applied at the inner wall of the glass tube to thereby emit visible rays.
The lamps 12 are arranged in parallel on the lamp housing 10. As shown in FIG. 2, the lamps 12 are arranged on the lamp housing 10 identically with the arrangement of the high-pressure electrode H and the low-pressure electrode L.
The lamp housing 10 prevents light-leakage of the visible rays emitted from each of the lamps 12 and reflects the visible rays proceeding to both a side surface and a rear surface of the lamps 12 toward the diffusion plate 14 to thereby improve light efficiency of the lamps 12.
The diffusion plate 14 makes the light emitted from the lamps proceed toward the liquid crystal panel 20 and make the light incident at an wide scope of angle. The diffusion plate 14 includes a transparent resin film having a light diffusion material coated on both sides thereof.
The optical sheets 16 lessen a visual angle of the light emitted from the diffusion plate 16 to thereby improve a brightness of a liquid crystal display and to decrease a power consumption.
Since in the LCD adopting the direct-below-type back light of the related art, the high-pressure electrode H and the low-pressure electrode L of the plurality of lamps 12 are identically arranged, the difference of a right/left color sense arises. That is, during manufacturing process of the lamps 12 used in the direct-below-type back light, the difference of the color coordinate (the difference of the color sense) is generated in a length direction in accordance with the length of the lamp.
Explaining this in full detail, as shown in FIG. 3, the manufacturing process of the lamps 12 used in a direct-below-type back light comprises preparing an organic solvent 32 mixed with the phosphors of each of red R, green G, and blue B contained in a vessel 30. And then, a lamp tube 34 is connected to a vacuum pad 36, and the organic solvent is imbibed in the vessel 30 through the lamp tube 34 by a vacuum pressure and the organic solvent 32 is applied to the lamp tube 34 by vacuum. Thereby, the organic solvent 32 is coated on inside wall. If the phosphor is coated on the inside wall of the lamp bulb 34, electrodes are installed in both side of opposing open parts and the open parts of both side are sealed to thereby complete a lamp.
Since the phosphor 38 is coated on the inside wall 34 of the lamp bulb by vacuum pressure, in accordance with the vacuum pressure and the length of the lamp bulb, the thickness of the lower part is thicker than that of the upper part. Accordingly, in accordance with the thickness of the phosphor 38 coated on the inside wall of the lamp bulb 34, the difference of the color sense between the high pressure electrode H and the lower pressure electrode L arises. Though the brightness of the back light having the phosphor of BaMgAl10O17:EU2+(BAM) has a brightness characteristic more than 10% of back light having the phosphor of SCA(Sr, Ba, Ca)10(PO4)Cl2:Eu, since the lamp is manufactured by the manufacturing process as described above, the difference of the right/left color sense according to the length of the lamp is generated excessively. On the other hand, the SCA of the phosphor has a defect that high price of optical sheet or brightness improvement of the panel is required.
Further, in the LCD employing the direct-below-type back light of the related art, high voltage and high frequency of AC wave is supplied to the lamps 12 and noise due to the high voltage AC wave is mixed with control signals or picture signals for driving the liquid crystal panel. For this reason, there occur the high voltage noise generated from inverter and ripple noise of wave form on the liquid crystal panel by the interference such as horizontal synchronization frequency of the liquid crystal panel. Accordingly, the ripple noise is appeared on pictures displayed on the liquid crystal panel of the LCD to thereby deteriorate the brightness characteristic.